Electrochemical sensor element with a porous reference gas accumulator

ABSTRACT

An electrochemical sensor element is described, in particular for determining the oxygen level in gas mixtures, having at least one measuring electrode ( 15 ) exposed to a measured gas, at least one reference electrode ( 17 ) exposed to a reference gas, at least one heating device ( 23 ), and one reference gas channel ( 21 ), through which the reference gas can be supplied to the reference electrode ( 17 ). The reference electrode ( 17 ) is connected to the reference gas via a volume provided with pores. The volume is formed in a layer between the reference gas channel ( 21 ) and the reference electrode ( 17 ).

[0001] The invention relates to an electrochemical sensor element, inparticular for determining the oxygen level in gas mixtures according tothe preamble of claim 1.

BACKGROUND OF THE INVENTION

[0002] Sensor elements of the generic type mentioned above are known.They are designed as planar sensor elements, which have, on a solidelectrolyte designed as a support, a first electrode exposed to themeasured gas and a second gas exposed to a reference gas. Furthermore,an electrical resistance heater is embedded in the support. A referencegas, which is in most cases made up of atmospheric air, is supplied tothe reference electrode via a reference gas channel integrated in thesupport. At the same time, the reference channel forms a gas chamberhaving a bottom surface matching the reference electrode in thereference electrode area, so that sufficient oxygen may reach thereference electrode. It is known from European Patent 125069 B1 that thewidth of the reference gas channel can be adapted to match the width ofthe electrode over its entire length for this purpose or two referencegas channels, with one electrode arranged in each, can run in a layerplane parallel to one another, with the two electrodes connectedtogether, form the reference electrode. The disadvantage of a widereference gas channel or a reference gas channel made up of two adjacentparts is that one part of the heating coil of the resistance heaterelement is always in the area of the perpendicular projection of thereference gas channel. This result in overheating of the solidelectrolyte in the area of the reference gas channel. In addition, awide reference gas channel provides poor heat transfer between theresistance heating element and the electrodes.

[0003] The method proposed in German Patent Application 19609323 A1, inwhich the reference gas channel is branched in the area of the heatingdevice, offers a possible remedy. However, in this case the referenceelectrodes must also be branched.

ADVANTAGES OF THE INVENTION

[0004] The sensor element according to the present invention having thefeatures named in claim 1 has the advantage that it allows improved heattransfer between the electrodes and the resistance heating element,resulting in uniform heat distribution. The porous layer also helpsrelieve mechanical stresses that occur at the edges where the referencegas channel and the adjacent solid electrolyte film meet, and which mayresult in stress cracks in the ceramic support. In bridging a widereference gas channel, the solid electrolyte film is bent, which resultsin additional mechanical stresses. Using the narrow reference gaschannel, excessive bending of the adjacent solid electrolyte film isavoided. Furthermore, due to the large-surface contact of the referenceelectrode with the adjacent porous layer, better adhesion of the latteris achieved, since the reference electrode remains pressed between theadjacent films during lamination. This is also true for the lead to thereference electrode, with its resistance also being thereby reduced.

[0005] Advantageous embodiments of the invention result from the otherfeatures named in the subclaims. It should be emphasized that thereference gas channel may have a slightly widened handgrip shape in thearea of the reference electrode. This allows oxygen exchange to beimproved, in particular in the case of low pore volumes. The effect ofthe reference atmosphere can be intensified by adding an oxygen-storingmaterial, for example, CeO₂, to the porous layer. This can be achievedby impregnating the porous layer or the porous electrode.

DRAWING

[0006] The invention is explained below with reference to theembodiments illustrated in the drawing.

[0007]FIG. 1 shows a cross section through the sensitive part of asensor element according to the present invention;

[0008]FIG. 2 shows a longitudinal section through the sensor elementalong line II-II of FIG. 1 according to a first embodiment; and

[0009]FIG. 3 shows a longitudinal section through the sensor elementalong line II-II of FIG. 1 according to a second embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0010]FIG. 1 shows a cross section through the measured gas side of asensor element. The sensor element is a component of a gas sensor (notshown) and is secured in a housing of the sensor and its sensitivesection is exposed to a gas to be measured. The sensor element is madeof a ceramic support 10 having a planar layer structure with a firstsolid electrolyte film 11, a second solid electrolyte film 12, and athird solid electrolyte film 13 superimposed on one another. Outer largesurface of first solid electrolyte film 11 has a measuring electrode 15and its inner large surface has a reference electrode 17. Measuringelectrode 15 is covered with a porous protective layer 19. Adjacent tofirst solid electrolyte film 11 is second solid electrolyte film 12,which has a narrow hollow cavity forming reference gas channel 21 in themiddle. Between second solid electrolyte film 12 and third solidelectrolyte film 13 an electrical resistance heating element 23 isarranged between two electrical insulating layers 25. Since electricalinsulating layers 25 are porous so they can absorb mechanical stressesdue to the different heat expansion coefficients of the materials used,a gas-tight solid electrolyte frame 27 is placed around insulating layer25. Electrical resistance heating element 23 is designed as a heatingcoil on the sensitive section of the sensor element.

[0011] On one narrow side of ceramic support 10, reference channel 21has a reference gas opening 29 and runs approximately to the oppositeend face of ceramic support 10, where reference gas channel 21 isclosed. In the embodiment of FIG. 2, reference gas channel 21 has auniform, for example, rectangular, cross section over its entire length.For example, the width of unsintered reference gas channel 21 is 0.4 mmto 0.8 mm, preferably 0.6 mm. The height of reference gas channel 21 isequal to the thickness of sintered solid electrolyte film 12, forexample, 0.4 mm. In the embodiment of FIG. 3, reference gas channel 21has a slightly widened section 31 in the area of reference electrode 17,so that reference gas channel 21 has a handgrip shape overall whenviewed from above. Reference gas channel 21 may also branch off in thearea of reference electrode 17.

[0012] Reference electrode 17, which has a flat shape in the plane ofceramic support 10, is covered with a porous layer 33 according to afirst embodiment. Porous layer 33, which is represented by a dottedsurface in FIGS. 2 and 3, is embedded between reference electrode 17 andthe adjacent second solid electrolyte film 12. First solid electrolytefilm 11 has a depression, for example, on whose bottom referenceelectrode 17 is placed with the porous layer filling the depression overreference electrode 17. Thus porous layer 33 spans reference gas channel21 in this area after films 11, 12, 13 have been laminated together. Thereference gas penetrating via reference gas opening 29 then diffuses viaporous layer 33 to reference electrode 17 positioned thereon. Thethickness of the porous layer is 5 μm to 200 μm, preferably 20 μm to 50μm.

[0013] In another embodiment for performing gas exchange with thereference gas, reference electrode 17 itself has a porous design.Furthermore, an embodiment may use one porous layer and one porousreference electrode. A suitable pore volume is required in order to forman appropriate reference gas chamber. This is achieved through thethickness of porous layer 33 and/or of porous reference electrode 17.

[0014] Good oxygen exchange can be achieved at reference electrode 17 byadding an oxygen-storing material, for example, CeO₂, to porous layer 33and/or porous reference electrode 17. The oxygen-storing material can beadded by impregnating porous layer 33 and/or porous reference electrode17. If the sensor element is operated as a concentration cell, referenceelectrode 17 can be supplied with sufficient oxygen by applying anelectric voltage to measuring electrode 15 and reference electrode 17.Thus an oxygen pumping effect is achieved in that oxygen is pumped frommeasuring electrode 15 to reference electrode 17. An additional pumpedinternal oxygen reference is thus formed on reference electrode 17.

1. An electrochemical sensor element, in particular for determining theoxygen level in gas mixtures, having at least one first electrodeexposed to a measured gas, at least one second electrode exposed to areference gas, at least one heating device, and one reference gaschannel through which the reference gas can be supplied to the referenceelectrode, characterized in that the reference electrode (17) isconnected to the reference gas channel (21) via a volume provided withpores.
 2. The sensor element according to claim 1, characterized in thatthe volume provided with pores is formed in a layer between thereference gas channel (21) and the reference electrode (17).
 3. Thesensor element according to claim 1 or 2, characterized in that a porouslayer (33) is formed next to the reference electrode (17), which isconnected to the reference gas channel (21).
 4. The sensor elementaccording to claim 1 or 2, characterized in that the volume is formed bythe p ores of a porous reference electrode (17),
 5. The sensor elementaccording to one of the foregoing claims, characterized in that anoxygen-storing material is added to the porous layer (33) and/or theporous reference electrode (17).
 6. The sensor element according toclaim 5, characterized in that the oxygen-storing material is CeO₂. 7.The sensor element according to one of the foregoing claims,characterized in that the measuring electrode (15) and the referenceelectrode (17) are connected as a concentration cell and an electricalvoltage is applied to them, which produces an oxygen pumping effect fromthe measuring electrode (15) to the reference electrode (17), so that anadditional pumped internal oxygen reference is formed in the volumeformed by the pores.